1. Field of the Invention
The present invention relates to a charging method of a rechargeable battery, such as a lithium ion battery, a lithium polymer battery, or the like.
2. Description of the Related Art
The market of lithium ion batteries or the like has been rapidly expanding because rechargeable batteries, such as, a lithium ion battery or the like, are widely used as power sources in various portable electronic apparatuses, such as, a notebook-type personal computer, a cellular phone, etc.
When charging a lithium ion battery, in view of managing the remaining capacity of the battery, it is important to reduce the variation in the amount of charged electric current in the fully-charged battery.
When charging a lithium ion battery, it is necessary to prevent the battery from being overcharged in view of safety during the charging. Thus, the charging voltage should not be unlimitedly increased. In a charging method of a lithium ion battery illustrated in FIG. 6, CVCC (Constant Voltage, Constant Current) charging control is performed. In the CVCC charging control, in a first step, the lithium ion battery is charged by allowing a constant current to flow between a positive terminal and a negative terminal of the lithium ion battery (constant-current charging: the charging voltage increases with the passage of time). Then, after the voltage between the positive and negative terminals of the lithium ion battery reaches a predetermined voltage value, the lithium ion battery is charged by applying a constant voltage between the positive and negative terminals of the lithium ion battery (constant-voltage charging: the charging current decreases with the passage of time) until the lithium ion battery reaches a fully charged state.
In FIG. 6, during a charging period from time t0 to time t1 (Constant Current (CC) period), the lithium ion battery is charged in a constant-current charging mode. During a charging period from time t1 to time t2 (Constant Voltage (CV) period), the lithium ion battery is charged in the constant-voltage charging mode. During the CC period, the charging current is constant. During the CV period, the charging voltage is constant.
In the CVCC charging control, when charging is complete, the total amount S of charged electric current in the fully-charged lithium ion battery is represented by the total area of the shaded region in FIG. 6 which is enclosed by a characteristic curve of the charging current and the vertical and horizontal axes. When the charging current becomes smaller than a predetermined charging current value Ic during the CV period, it is determined that the lithium ion battery is fully charged, and the charging is stopped. The current value Ic at which the charging is stopped is set so as to be about {fraction (1/10)} to {fraction (1/15)} of the charging current value employed during the CC period, i.e., 0.07 C (e.g., 100 mA in the case of a battery having a capacity of 1500 mAh).
During the CV period, the shape of the characteristic curve of the charging current always results in substantially the same shape so long as the temperature is maintained at a certain value, while the charging current value decreases with the passage of time. Thus, if the temperature is constant, the total amount S of electric current charged in the lithium ion battery always substantially the same at the end of each charging operation as shown in FIG. 6, i.e., the variation in the total amount S of charged electric current decreases.
However, when the temperature of the lithium ion battery varies as shown in FIG. 5, the internal resistance of the lithium ion battery also varies, and accordingly, the shape of the characteristic curve of the charging current for the lithium ion battery is changed. In the case where the current value Ic at which it is determined that the battery is in a fully charged state (hereinafter, xe2x80x9cfull charge determination current valuexe2x80x9d) is constant, the times at which the lithium ion battery becomes fully charged and the charging operation is completed (hereinafter, xe2x80x9ccharging completion timexe2x80x9d) for three different temperatures of the lithium ion battery, T1, T2, and T3 (T1 less than T2 less than T3) are t1, t2, and t3 (t3 less than t2 less than t1), respectively. That is, as the temperature of the lithium ion battery decreases, the time at which the lithium ion battery becomes fully charged is delayed. In this case, the total amounts of charged electric current, S1, S2, and S3, of the fully-charged lithium ion battery for the temperatures T1, T2, and T3 have a relationship of S1 less than S2 less than S3. That is, as the temperature of the lithium ion battery becomes higher, the amount of electric current charged in the lithium ion battery increases. This is due to the fact that, when the temperature of the lithium ion battery is low, the internal resistance of the lithium ion battery is increased, and accordingly, the total amount S of electric current charged in the lithium ion battery is decreased. On the other hand, when the temperature of the lithium ion battery is high, the internal resistance of the lithium ion battery is decreased, and accordingly, the total amount S of electric current charged in the lithium ion battery is increased.
Thus, when c harging the lithium ion battery, if the temperature of the lithium ion battery varies, it is difficult to suppress the variation in the amount of electric current charged in the fully charged battery.
As described above, in the CVCC charging control of the lithium ion battery, the full charge determination current value Ic is maintained at a constant value as shown in FIG. 6 even if the temperature of the lithium ion battery varies. Insucha full charge state detection method, the amount of electric current charged in the battery varies according to the variation of the temperature of the battery. Thus, error may be caused in the management of the remaining capacity of the battery.
According to one aspect of the present invention, a charging method of a rechargeable battery includes steps of: charging the rechargeable battery with a constant current; charging the rechargeable battery with a constant voltage or with a current having a pulse waveform; and when a charging current is decreased so as to be equal to or lower than a prede termined current value, or when an average charging current f or a single pulse of the current having a pulse waveform is decreased so as to be equal to or lower than a predetermined current value, determining that the rechargeable battery has been fully charged and stopping the charging of the rechargeable battery, wherein, in the step of determining that the rechargeable battery has been fully charged, a temperature of the rechargeable battery is measured, and the predetermined current value at which the charging of the rechargeable battery is stopped is adjusted according to the measured temperature.
In one embodiment of the present invention, in a case where the temperature of the rechargeable battery varies, the predetermined current value is set according to the temperature of the rechargeable battery such that the total amount of electric current charged in the rechargeable battery is always the same for different temperatures of the rechargeable battery upon completion of charging the rechargeable battery.
In another embodiment of the present invention, the predetermined current value is adjusted according to the temperature of the rechargeable battery such that the total amount of electric current charged in the rechargeable battery is always the same upon completion of charging the rechargeable battery.
Thus, theinvention describedhereinmakespossible the advantages of providing a CVCC charging method by which, in a charging operation of a lithium ion battery, even when the battery temperature varies, the total amount of electric current charged in the battery is always results the same without fail.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.